Electron-emission material and method of preparation



Patented May 22,1928. I

UNITED STATES,PATENTTOFFICE.

JOHN -WESLEY MABDEN, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTING-HOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

ELECTRON-EMISSION MATERIAL AND METHOD OF PREPARATION.

No Drawing.

This application is a continuation-impart of my copending applicationSerial No. 504,154, filed Sept. 29,1921, production of rare metals andalloys thereof and assigned to the \Vestinghouse Lamp C0mpany.

This invention relates to electron-emlssion material and the preparationthereof and more particularly to the em loyment of alloys of aluminumand meta s such as thorium, uranium, zirconium and the like foractivating such material and for other purposes.

An object of my invention is the preparation of activated filamentarymaterial by 1 ncorporating :efiicient metallic electron-em ssionmaterial into refractory metal, said electron-emission material beingfirst combined with another metal as a carrier and protector of theelectron-emission material. Another object of my invention is theemployment of alloys of aluminum with refractory rare metals foractivating material for electron-emission purposes and the like.

A further object of my invention is the activation of metals of thetungsten class including tungsten, molybdenum, platinum, tantalum andthe like by incorporatmg therewith an alloy of aluminum and anelectron-emission material such as thorium, uranium, titanium, zirconiumor the like and subse uently eliminating the aluminum there rom byvaporization.

A still further object of my invention is the preparation of alloys ofaluminum and refractory rare metals such as thorium, ura-, nium,zirconium, titanium and the like for application to an element of anelectron device, whereby activation of an electron-emitting filamenttherein may be accomplished by the rare metal, simultaneously with aclean-up action by the aluminum.

Other objects and advantages of the invention will become apparent asthe description proceeds. I

In the copending application of Marden and Rentschler Serial No.583,377, filed August 21, 1922."rare metal alloys and method of prearation and assigned to the Westinghouse llamp Company, is disclosed amethod of preparing activated molybdenum by incorporating a smallproportion of metallic thorium therein prior to pressing, sintering andworking the same to the desired form. The method disclosed. has givenood results, but it entails the use of metallic thorium Applicationfiled April 11, 1923. Serial No. 631,366.

which is not only difficult and expensive to prepare, but readilyoxidizes and is easily contaminated by moisture, .nitrogen and othermaterials.

In the copending application of C. T. Ulrey, Serial No. 577,021, filedJuly 24, 1922,

manufacture of electron-emitting devices.

.is satisfactory, but. likewise entails the useof metallic thoriumwhich, as before noted, is expensive, difficult to prepare andreadilycontaminated by other substances.

According to my invention, I accomplish the results obtained accordingto the aforementioned applications, but instead of using metallicthorium, I propose to use an alloy, solid-solution, mixture or compoundof thorium or other material with high electronemitting properties, suchas uranium, zirconium or the like, with aluminum, which serves as aprotector, of the thorium or other rare metal, from contamination andholds it in a form in which it may be conveniently utilized for thepurposes desired. The alloy of the electron-emitting metal and aluminummaybe prepared in a manner similar to that disclosed in my copendin'gapplication before referred to, Serial No. 504,154, filed September 29,1921, production of rare metals and alloys thereof and assigned to theWestinghouse Lamp Company.

After preparing the desired aluminum alloy, according to saidapplication, the same may be powdered and mixed in the desiredproportion with the' selected refractory metal to be activated, forexample, molybdenum. The mixture may then be pressed into the form of acake or slug,

tered in a vacuum or heat-treated to cause the particles to coalesce toform a homohigh enough tempreferably square in cross sectlon. It maythen be sin-' Instead of producing activated material in such manner, anelectron-emitting cathode may be activated in place in a radio tube orthe like, by applying to an adjacent plate or anode in said tube, acoating of powdered thorium aluminide or other alloy or mixture ofaluminum with a metal having good electron-emissivity.

Said material may be applied by mixing the same with a suitable binder,such as an amyl-acetate solution of nitrocellulose, and painting themixture upon the anode or, if desired, directly on the cathode. Insteadof applying the alloy by painting, the same may be formed of a strip orclip and fastened to said plate or anode in any suitable manner, as bypasting, crimping or welding.-

' process may be employed for preparing acpared tivated tungsten, orother metal of the tungsten class.

The thorium-aluminum alloy may be preaccording to my aforementionedapplication Serial No. 504,154, ofwhich this application is acontinuation-in-part or, briefly, as follows.

A halide salt of thorium, for example, thorium-ammonium-chloride isprepared, preferably by the recess disclosed in my copending applicationSerial No. 498,397, filed September 3 1921, preparation of metals andtheir halides and assigned to the Westinghouse Lamp Company. -Thatprocess involves the treatment of the hydrated salts of the rare metalswith a displacing substance, for example, ammonium chloride added to asolution thereof. The solution may then be evaporated to dryness toeliminate the water and part of the ammonium chloride.

The residue, which is thorium-ammoniumchloride, may be placed in agraphite crucible and covered with a previously fused and cooled mixtureof sodium and potassium chloride or fluoride or other halogen alkalicompounds. The crucible, with the mixture therein, may be placed in afurnace and the temperature raised until the mixture is fused to a clearliquid.

In order to prevent oxidation, 9. slow current of carbon tetrachloridevapor or other desired manner to recover the thorium-aluminum alloy oraluminidc. It is deemed preferable to purify the powder by treating witha fairly strong solution of caustic soda to dissolve out the excess ofaluminum not alloyed with the thorium.

The residue, which is thorium aluminide (Th Al may then be washed withwater and treated with dilute acid, for example, hydrochloric acid, toremove all the caustic soda and acid-soluble materials from the alloy.After filtering and drying, the aluminide may be used for activatingpurposes, as will be subsequently described, for hardenlng aluminum byformin aluminum alloys containing'small proportlons of refractory raremetals or for any desired purpose.

The powdered alloy or aluminide may be mixed in the desired proportionwith pow-- dered molybdenum or the like. In order to repare activatedmaterial or material havmg high electron-emissivity, it is onlynecessary to use a small proportion of the' thorium alloy with amolybdenum, tungsten or other refractory metal powder which may be used.For example, from V; to 1%, by weight, of the alloy 1s sutlicient togive good activation. Such mixture may then .be pressed into a cake orslug, preferably square in cross section, and considerably longer thanthe cross sectional dimensions thereof. Slugs for experimental work 3inches long and inch square in cross section, may be used, but, inpractice, it is preferable to use slugs about inch to inch square and 8inches long.

The slug may be sintered to a high temperature in a high-frequencyinduction furnace, that is, for example, the pressed slug may be placedon a sheet of molybdenum or other refractory metal and inserted in asintering furnace of the type described in the copending application ofMarden and Rentschler Serial No. 488,229, filed July 28, 1921, a processof m anufacturin metals and their uses, and assigned to the estinghouseLamp Company.

Such sintering is optional and 1s recommended for slugs of small sizeonly, or where such have been formed too weak for the heat treatment,later described, without sintering. If the material has beensufiiciently sintered and' the of the same maintained, by causinthe'sintering to beaccom-' plished by a hig vacuum, the current may thenbe shut offend the resultant product permitted to cool within thefurnace while maintaining the high vacuum. v r

A pressed slug either sintered,',or the optional sintering operationhaving been emitted, maythen be heat-treated, at a tem- 'perature nearlyhigh enough to melt it, in a specially-constructed furnace, for example,

i such as that described and claimed in the copending application ofRentschler and Marden Serial No. 488,230, filed Julyv 28, 1921, furnacesand assigned to theWestinghouse Lamp Company. Any furnace wherein theslug may be heatedin an inert environment, such as a high vacuum, tonearly the melding point thereof, may be employed.

The resulting alloywill be found to be strongly coherent, dense, toughand less liable to crack on-unequal heating and cooling and to have lostentirely all of its brittleness, so that, when it is heated red-hot, it-

may be readil swaged, rolled, hammered or drawn to ren er it ductile.The slug may be rolled to size or swaged hot until round incross-section and then drawn down to the desired size of wire, in amanner well known for working molybdenum, if such is used, or for workmgtungsten or other refractory metal, if such is used, instead ofmolybdenum. r

The sintering or heat treatment previously described may be conducted atsuch high temperature as to eliminate the aluminum content bydistillation or vaporization thereof. The thorium aluminide, or otheralloy used, will not dissociate until the slug has been substantiallyentirely rid of gases, which prevents the thorium,- after the aluminumhas been vaporized therefrom, from gas contamination, whereby it isleftin a metallic condition alloyed withthemolybdenum, tungsten orwhatever refractory metal, the thorium-aluminum alloy has been added to.Wire formed from such an alloy, is an eflicient electron-emissionmaterial, in fact as.

good as pure thorium, even though the thormm content may be ver small.The aluminum content has a uri ying and scavenging action on the prouct, whereby an alloy is obtained which is more free from oxides andother deleterious impurities than those heretofore employed.

. Activation by means of thorium aluminide or other thorlum-aluminumalloy may be effected, in an electron tube or the like, by

applyin the same upon the plate or anode therein, y mixin the same in apowdered form with a suite le vehicle or binder, such as an amyl-acetatesolution of nitro-cellu lose, and painting the mixture upon the plate.An alternative method may consist in applying the aluminide to the plateor plate may then be heated to a bi h temperature in any convenientmanner, or example, as described in the aforementloned applicatlon istran'sferre or sputtered from the plate to the electron-emittingfilament, to cause the same to become activated. The aluminum content,in this instance, may serve a double, function of preserving the thoriumfrom contamination until it is desired to use anodein the form of a clipor strip fastened thereto in any suitable manner.

of C. T. Ulre whereby the thorium content the same for activatingpurposes and, also metal is usedmerely as a hardener and strengthenerin. the aluminum, togive the same desirable properties for castingswhich could be used in automobiles, aeroplanes and other devices needinglight metals of high tensile strength. and where the parts could becast.

The-desired alloy may be prepared as herealuminum is usedso that theproportionof the rare metal, for example, zirconium, is

tofore described, although preferably more relatively small. Aluminumholds about 8% of zirconium, after which crystals, havingabout 30%aluminum in them and corresponding to the formula Zr,Al separate fromthe mass. An 8% zirconium-aluminum alloy has a Brinellhardness of about54, a

Shore hardness of about 12, an elastic limit of 13,500 pounds per -suare inch and a tensile strength of about'2 ,000 pounds per square inch.This is about the same tensile strength and hardness as acopper-aluminum alloy, having perhaps about 10 to 12% copper. A 4%zirconium-aluminum mixture or alloy has-a hardness and tensile strengthcor, relsponding to about a 6% copper-aluminum a 0y.

Although .I have described what I now consider to be the preferredmethods of practicing my inventlon, in connection. with certain specificalloys or mixtures, it is to be noted that the-alloys mentioned aremerely illustrative and that my invention is adapted for alloys ofaluminum with the refractory rare metals for electron-emission purposesor 7 general use.

What is claimed 1s:

1. The process of activating refractory metal for electron-emissionurposes com prising incorporating a sma proportion of a powderedaluminum alloy of electrona desired proportion to cause the particlesthereof to be firmly welded together, said heat-treating operation beingconducted in a vacuum.

3. The process of alloyin or. forming a coherent ductile alloy ofthorium and mol bdenum comprising mixing powdered t 10- rium-aluminideand powdered molybdenum in the desired proportion, pressing the same inthe form of a slug and heat-treating until the aluminum is eliminatedtherefrom and the particles of thorium and molybdenum become firmlywelded together.

4. The method of making alloys or mixtures comprising mixing an alloy ofthorium and aluminum in powdered form with powdered molybdenum, pressingthe mixture into the form of a slug and heat-treating to vaporize thealuminum therefrom and cause the particles of the remaining alloy toweld firmly together said heat-treating operation being conducteinaninert environment.

5. The method of producing refractory I alloys for electron emissionpurposes whic comprises activatmg a filament b .the addition of an alloyof aluminum an a metallic electron emission materialand purifying byheating the filament in a vacuum to eliminate the aluminum therefrom.

6. The method of activatin refractory metal filaments consisting in adin thereto an alloy of thorium and a'vaporizable metal and heating thefilament to eliminate the vaporizable metal.

7 .The method of activating refractory metal filaments comprisingassociating therewith, an alloy of thorium and aluminum and heating thefilament to eliminate the aluminum therefrom.

In testimony whereof, I have hereunto subscribed my name this 10th dayof April 1923. i

' J OHN WESLEY MARDEN.

